1. Field of Invention
This invention pertains to a light guide for use in association with a detector array for use in imaging applications such as X-ray imaging, fluoroscopy, positron emission tomography (PET), single photon emission computed tomography (SPECT), computed tomography (CT), gamma camera and digital mammography systems. More particularly, the present invention pertains to a continuous light guide for use in association with a plurality of detector arrays.
2. Description of the Related Art
In the field of imaging, it is well known that imaging devices incorporate a plurality of scintillator arrays for detecting radioactivity from various sources. Each scintillator array is comprised of a plurality of scintillation crystals which interact with incident high energy photons. In Positron Emission Tomography (PET), the scintillators are provided for detecting photon energy peaks at 511 keV. At least a portion of the energy from the photons is absorbed, depending on the atomic number density (N) and effective atomic number (Z) of the scintillator. As the product (N×Z) increases, the absorption probability in the scintillator increases. Absorbed energy in the crystal is converted to lower energy scintillation photons. The scintillation photons are collected by an array of photodetectors comprised of at least one light-sensing element, such as a photomultiplier tube (PMT) or a solid state photo detector. Typical solid state photo detectors include, for example, avalanche photodiodes (APDs), PIN photodiodes and the like.
It is well known to interpose a light-transmitting media, or light guide, between the scintillator arrays and the photo-detector array. The light guide is tuned using several known methods. In one such method, slots of various depths are defined in the light guide to channel the scintillation light and enhance the positioning information of the crystals. In a further such method, a reflective material such as paint is applied at various paint levels on the side surfaces of the scintillation crystals in order to channel the light to obtain uniform flood source images.
Conventional nuclear imaging systems consist of detector blocks, each of which is a modular unit in square or rectangular scintillator arrays with the dimension of M×N crystals. Alternatively, gamma cameras in nuclear imaging have a continuous scintillator, rather than pixilated, and a continuous light guide. The physical properties of the scintillator crystal play an important role on the performance of the detector system. High stopping power, short decay time and high light yield are the most desirable features for a scintillator. Crystals that have a relatively longer decay time limit the count rate capability when the imaging system is exposed to high levels of activity in the field-of-view (FOV).
It is common practice, when constructing scintillator arrays composed of discrete scintillator elements, to pack the scintillator elements together with a reflective medium interposed between the individual elements fully covering at least four sides of the scintillator element. The reflective medium serves to collimate the scintillation light to accurately assess the location at which the radiation impinges upon the detectors. The reflective medium further serves to increase the light collection efficiency from each scintillator element as well as to control the cross-talk, or light transfer, from one scintillator element to an adjacent element. Reflective mediums include reflective powders, reflective film, reflective paint, or a combination of materials.
Conventionally, scintillator arrays have been formed from polished crystals that are hand-wrapped in reflective PTFE tape and bundled together, or alternatively, glued together using a white pigment such as MgO, BaSO4 or TiO2 mixed with an epoxy or RTV. In a further alternative, the crystals are painted with tuned reflective paint partitions and glued with an epoxy to form the block.
Another approach utilizes individual reflector pieces that are bonded to the sides of the scintillator element with the aid of a bonding agent. This process requires iterations of bonding and cutting until a desired array size is formed.
Other devices have been produced to form an array of scintillator elements. Typical of the art are those devices disclosed in the following U.S. Patents:
Pat. No.Inventor(s)Issue Date3,919,556W. H. BerningerNov. 11, 19753,936,645A. H. IversonFeb. 3, 19764,749,863M. E. Casey et al.Jun. 7, 19884,914,301Y. AkaiApr. 3, 19904,982,096H. Fujii et al.Jan. 1, 19915,059,800M. K. Cueman et al.Oct. 22, 19915,319,204W. H. WongJun. 7, 19945,453,623W. H. Wong et al.Sep. 26, 19955,753,917J. C. EngdahlMay 19, 19986,087,663C. Moisan et al.Jul. 11, 20006,292,529S. Marcovici et al.Sep. 18, 20016,462,341G. MuehllehnerOct. 8, 2002
Of these patents, the '645 patent issued to Iverson discloses a radiation sensitive structure having an array of cells. The cells are formed by cutting narrow slots in a sheet of luminescent material. The slots are filled with a material opaque to either light or radiation or both. The '800 patent issued to Cueman et al., discloses a similar scintillator array wherein wider slots are formed on the bottom of the array.
Wong, W. H. et al., in “An Elongated Position Sensitive Block Detector Design Using the PMT Quadrant-Sharing Configuration and Asymmetric Light Partition,” IEEE Transactions on Nuclear Science, Vol. 46, No. 3, 542–545 (1999), discloses a block design wherein seven (7) monolithic BGO slabs are painted with light-blocking reflective patterns on their boundaries. The slabs are then glued together to form a block. The block is then cut orthogonally with respect to the glued seams and painted and glued again in like fashion. A 7×7 array is thus defined. The reflective patterns are unclear from the disclosure, but appear to be defined only for the cut portions analogous to the '863 patent discussed below, such that the reflective areas increase toward the central portion of the array.
As discussed, most of the aforementioned methods also require a separate light guide attached to the bottom of the detector array to channel and direct the light in a definitive pattern on to a receiver or set of receivers such as photomultiplier tubes or diodes. The '863 patent issued to Casey et al., discloses a two-dimensional photon position encoder system and process which includes a detector for enhancing the spatial resolution of the origin of incident photons of gamma rays. A plurality of scintillator elements interact with the incident photons and produce a quantifiable number of photons which exit the scintillation material members. A tuned light guide having a plurality of radiation barriers of predetermined lengths define slots which are operatively associated with one of the scintillator material members. The slots serve to enhance the predictability of the statistical distribution of photons along the length of the slotted light guide. A detector detects the distribution of the photons at pre-selected locations along the length of the slotted light guide.
Berninger, in the '556 patent, discloses a scintillator on which impinge incident collimated gamma rays. Light pulse output from the scintillator is detected by an array of photoelectric tubes, each having a convexly curved photocathode disposed in close proximity to the scintillator.
In the '204 (Wong) and '623 (Wong et al.) patents, a PET camera is disclosed as having an array of scintillation crystals placed next to other arrays either around or on opposing sides of a patient area. An array of light detectors is positioned next to four adjoining quadrants of four respective scintillation crystal arrays to detect radiation emitted from the four quadrants of each array. The crystals within the arrays are selectively polished and selectively bonded to adjoining crystals to present a cross-coupled interface which can tunably distribute light to adjoining light detectors. The crystal arrays are formed by optically bonding slabs of crystals into a “pre-array” and then cross-cutting the “pre-array” from one or more sides to form the final array. Wong et al. disclosed that the grooves may be optically treated, such as with white reflective fillers, for further optical control within the array.
Engdahl, in his '917 patent, discloses a scintillation camera including a scintillation crystal assembly having multiple crystal layers for interacting with various photon energy levels. The camera is disclosed as performing imaging of conventional nuclear medicine radioisotopes as well imaging of high energy isotopes used in PET applications. The multiple crystal layers have the effect of doubling the sensitivity of the camera to high energy photons, while retaining the performance characteristics needed for conventional low energy photon imaging. According to one embodiment disclosed by Engdahl, the '917 device includes a scintillation crystal having a first layer composed of NaI(T1) and a second layer composed of CsI(Na). A collimator is provided for collimating photons incident on the crystal. An array of photomultiplier tubes is provided for detecting and localizing scintillation events within the crystal. The photomultiplier tubes are mounted on a glass lightpipe.
In the '341 patent, Muehllehner discloses a positron emission detection scanner including a first plurality of detecting elements arranged in a first two-dimensional geometrical array. The detecting elements are provided for communicating light from a scintillation event. A light guide is provided for receiving light from the scintillation events from each of the detecting elements, and then transmitting the light to an array of photodetectors arranged in a second two-dimensional geometrical array, the alignment of the light sensing members being independent of the detecting elements.
Moisan et al., in the '663 patent, disclose light guides capable of encoding the transverse and longitudinal coordinates of light emission induced by the interaction of photons in an array of a plurality of the light guides. Each light guide has at least two discrete crystal segments adjacently disposed along a common longitudinal axis of the light guide. Between adjacent segments is a boundary layer having less light transmission than the light transmission of the crystal segments. A light absorbing mask is provided to increase light adsorption in a segment. Photons entering the light guide cause the emission of scintillation light which is delivered in different and resolvable quantities to light sensing devices.
Also of interest is Anger H. “Scintillation camera”, Reviews of scientific instrumentation, 29(1):27–33, 1958.